1. Field of the Invention
The present invention relates to gaskets for sealing joints, such as bolted flange joints, in fluid conduits, and particularly gaskets which can be readily centered in such joints.
2. Description of Related Art
In the past many systems and designs have been proposed to center annular gaskets on bolted flanges. Centering of these gaskets is considered necessary for at least two reasons: (1) to assure a proper seal by retaining the gasket in the proper sealing orientation during installation; and (2) to avoid disruption of the flow of fluid inside the fluid conduits after installation, which may produce many undesirable conditions, such as non-laminar flow, unwanted losses in pressure, or the possible accumulation of fluid contaminants.
Two basic geometries have emerged as the primary commercially available solutions to this problem-ring gaskets and full face gaskets. Both types of gaskets use the bolts of the flange to help locate the gasket. Unfortunately, due to excessive tolerances in the positioning of the bolts in such joints, neither of these techniques can be considered particularly accurate.
Ring gaskets are designed so that their outer diameter (OD) makes contact with all bolts when the bolts are centered in bolt holes of the flange. By properly proportioning the center hole of the gasket relative to the bolt holes, this method should approximately center the gasket in place.
Full face gaskets are designed with bolt holes of the same general size and geometric layout as the flanges. The gasket is then mounted in place with the joint mounting bolts holding the gasket into place. However, in order to ease mounting, the bolt holes in the gasket are generally larger than the bolts themselves, introducing additional inaccuracy in the mounting process. This deficiency may be counteracted in some part by the fact that the gasket extends beyond the mounting bolts to the exterior of the conduit, allowing for some fine-tuning of the centering of the gasket if care is taken to make sure the gasket is completely even with the exterior of the conduit before final mounting of the gasket occurs.
A number of further solutions have been proposed to improve the centering of gaskets. Examples of various systems using mounting bolts to help mount and/or center gaskets are shown in: U.S. Pat. No. 605,891 issued Jun. 21, 1989, to Merwarth; U.S. Pat. No. 695,174 issued Mar. 11, 1902, to Roller; U.S. Pat. No. 1,942,704 issued Jan. 9, 1934, to Hubbard et al.; U.S. Pat. No. 3,480,301 issued Nov. 25, 1969, to Kroening; U.S. Pat. No. 3,781,043 issued Dec. 25, 1973, to Hagmann; U.S. Pat. No. 4,002,344 issued Jan. 11, 1977, to Smith; U.S. Pat. No. 4,436,310 issued Mar. 13, 1984, to Sawabe et al.; and U.S. Pat. No. 4,522,536 issued Jun. 11, 1985, to Vidrine.
A drawback with any system using bolts to help center the gasket is the excess tolerance often permitted for the bolts. Thus, only partial centering is achieved by using the bolts as guides since all standard flanges have bolt holes slightly bigger than the bolts. During installation the bolts move position within the bolt holes, and thus make it virtually impossible to center each bolt on its bolt hole to assure precise centering. Gaskets manufacturers and users having recognized this occurrence have specified in new ANSI B16.5-1988 standards gaskets' inner diameters somewhat bigger than the pipe's inner diameter (ID) to prevent gasket material from intruding into the flow even when the gasket is not perfectly centered.
The dimensions specified in the ANSI B16.5-1988 standards ensure that the ID of the gaskets can be misplaced by a maximum linear displacement of 0.12 inches for full face gaskets and 150 lb. rated flanges, assuming the gasketing material does not deform. This is the amount of space a bolt can move off center within its bolt hole, plus the amount of space the gasket can move off center from the bolt. When the ID of the gasket is the same as the ID of the flange, up to 30% of the pipe opening can be blocked by the gasket material in the smallest size, and about 48% of the circumference can become a crevice.
In most industries, it generally does not matter if the ID of the gasket is bigger than the ID of the flange, creating a crevice at the juncture. However, in certain applications, such as conduits carrying some ultra-pure materials or conduits requiring high speed laminar flow, any crevices or intrusions into the flow are critical. In high purity fluid transportation, crevices or protrusions can trap bacteria or contaminants which are later released into the mainstream flow, contaminating the rest of the system in a cyclical fashion. As a result, many steps have been taken in high purity industries to reduce protrusions and intrusions within the piping system's internal surfaces even to the microscopic level. In fact, piping pore size is regularly inspected by Scanning Electroll Microscopy (SEM) in some applications to guarantee a maximum value.
Since state of the art systems today measure contaminants in the parts per billion range, extremely small crevices or protrusions can be critical to the performance of the system. In these systems a way to precisely center the gasket so that the ID of the gasket exactly meets the ID of the flange is very desirable, but up to now not available.
Another approach which has been attempted is to use external tabs to assist in aligning and retaining the gasket in a centered position. Such devices are shown in U.S. Pat. No. 1,731,404 issued Oct. 15, 1929, to Wetherill as well as U.S. Pat. No. 4,522,536. Although these centering methods may provide some assistance on a macro-scale, they do not address the concerns of how to precisely center a gasket within joints demanding exact tolerances.
A further problem that has emerged in high purity applications and the like is the growing use of raised flange gaskets employing mounting bolts spaced apart from the flange sealing surfaces. Often these joints are connected using separated backing plates or similar devices pulled toward one another using long mounting bolts. For these applications it is often quite difficult to hold and center between sealing surfaces a ring gasket having the same geometry as the sealing surfaces. This difficulty is due to restricted access due to a relatively small gap between backing plates. This problem is compounded by the lack of bolts within the sealing surface to provide loose alignment and stable positioning of the gasket prior to final centering.
At present, the best available solution to ease mounting in raised flange joints is to use a modified full face gasket that extends well beyond the sealing surfaces to join with the mounting bolts. Although this is helpful for loose mounting of the gasket between the backing plates, precise alignment is significantly handicapped since the exterior of the conduit is difficult to use as a gauge for final positioning (i.e., in this case, the edge of the gasket extends beyond the external mounting bolts, providing no surface in direct contact with the gasket to guide final alignment). Further complicating centering in certain raised flanges is the fact that some backing plates are also loosely mounted, again providing excessive tolerances to hinder accurate gasketing centering. Finally, this method of mounting tends to be unduly wasteful of gasket material, with a substantial amount of material providing no sealing or other function.
Another approach in these applications is to use a ring gasket larger than the sealing surfaces that can be rested upon lower external mounting bolts for initial gasket placement. While this approach assists in initial gasket placement, it suffers from the same deficiencies as using a full face gasket in these applications, including difficulty in judging exact final gasket placement and needless use of gasket material external to the sealing surfaces.
Accordingly, it is a primary purpose of the present invention to provide a gasket that can be accurately and easily centered within a joint.
It is another purpose of the present invention to provide a gasket that eases initial positioning and final alignment of a gasket within raised flange gasket applications.
These and other purposes of the present invention will become evident from review of the following specification.